The present invention is directed generally to gas generating compositions, methods of producing them, and devices incorporating them, and more particularly, to high impetus, high burn rate gas generating compositions for use in automotive seatbelt pretensioners and other suitable applications.
Gas generating compositions used in automobile safety restraint systems must satisfy several important propellant criteria. These applications require compositions that generate gas with very high mass flow rates, have high thermal stability, and result in combustion products that do not contain excessively harmful gases or excessive solid particulates. An increasing focus on reduction of the toxicity of propellants and their combustion products has resulted in the progressive replacement of azide-based propellants (which were previously the standard airbag gas generants) with pyrotechnic-based formulations, which possess lower toxicity and better performance.
Pyrotechnic gas generant compositions are usually composed of a heterogeneous blend of one or more discrete fuel sources, such as hydrocarbons, tetrazoles, nitramines, guanidines, dicyandiamide, and other NHO containing compounds, which are mixed with one or more oxidizers, such as metal oxides, nitrates, and perchlorates, in varying quantities to produce a desired gas output with relatively benign combustion products, specifically N2, H2O, and CO2. Such compositions may also include burn rate catalysts to enhance the burn rate, binders for improved mechanical properties, and processing aids for easier processability.
Airbags, pretensioners, and similar applications generally mandate non-toxic combustion products, high thermal stability and durability, and a compact and economical overall assembly. The design criteria pertaining to applications such as seatbelt pretensioners, however, differ in some respects from those pertaining to airbags. The primary requirements for the propellant in a pretensioner are high impetus, and a high burn rate with low pressure exponent. Because the pretensioner's piston must be actuated very quickly (generally in less than seven milliseconds) and efficiently, the gas generant must produce a high energy output that tends to require a higher combustion temperature. While it is generally desirable to reduce combustion temperature in airbag applications, this is not a significant constraint in applications such as seatbelt pretensioners because the resulting combustion products are largely contained within the pretensioner's housing rather than being significantly expelled into the environment aspirated by the vehicle occupant, and because a large amount of the heat produced by the burning of the gas generant in a seatbelt pretensioner is converted into kinetic energy in actuating the pretensioner.
Single-base propellant has been a standard propellant in pretensioner gas generant compositions, however, it is associated with the drawbacks of lower thermal stability and high carbon monoxide combustion products. Consequently, there is a need for improved gas generant compositions for use in applications such as pretensioners.
In the context of automotive airbags, it has been known to use 5-aminotetrazole (5-AT) as a high energy clean-burning propellant ingredient, and also in conjunction with iron oxide as a combustion catalyst and a slag forming agent. A. Helmy and W. Tong, Thermal Decomposition of 5 Amino Tetrazole Propellant, 36th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, AIAA Publication No. 2000-3330, incorporated herein by reference.
U.S. Pat. No. 5,883,330 to Yoshida discloses a gas generant composition consisting essentially of azodicarbonamide (ADCA), an oxidizer, and a burning catalyst preferably in an amount of 0.2 to 10 wt. %.
U.S. Pat. No. 6,019,861 discloses a gas generating composition with a fuel in an amount of 15–30 wt. % and comprising 5-AT possibly with an addition of ADCA or ammonium oxalate, phase-stabilized ammonium nitrate (PSAN) oxidizer in an amount of 35–80 wt. %, and silicon powder having a particle size of 2–100 microns preferably in an amount of 0.5–7 wt. % and for an allegedly critical but unspecified purpose.
U.S. Pat. No. 6,074,502 to Burns et al. discloses a gas generant composition with a primary fuel such as 5-AT preferably in an amount of 9–27 wt. %, a secondary fuel in an amount of 1–15 wt. % and comprising ADCA or hydrazodicarbonamide, PSAN oxidizer in an amount of 55–85 wt. %, and an optional burn rate modifier in an amount of 0–10 wt. % and selected from a variety of possibilities including alkali metals.
U.S. Pat. No. 6,361,630 also discloses the use of a non-azide nitrogen containing organic fuel such as 5-AT or ADCA preferably in an amount of 15 to 35 wt. %, an inorganic salt oxidizer, a metal organic coolant, and optionally a burn rate modifier such as iron oxide in an amount of 1 wt. % in the illustrative examples.
U.S. Patent App. Pub. No. 2003/0015266 to Wheatley et al. discloses the preferred use of an azodiformamidine dinitrate fuel, a co-melt of silver nitrate and potassium nitrate, an auxiliary fuel such as 5-aminotetrazole nitrate, and “a powdered metal or metal oxide as a combustion catalyst to speed the decomposition reaction and also as a combustion aid to facilitate the ignition of the primary propellant or gas generant,” which metal or metal oxide powder includes “those based on iron, aluminum, copper, boron, magnesium, manganese, silica, titanium, cobalt, zirconium, hafnium, and tungsten,” with a particularly preferred example given as NANOCAT® superfine iron oxide material preferably having an average particle size of 2 nm, a specific surface density of about 250 m2g and a bulk density of 2 to about 5 wt. %. Wheatley also discloses an optional ignition accelerator/augmenter/enhancer in the form of a graphite powder preferably having an average particle size of 40 microns and in an amount of 0.5 wt. % to 1.5 wt. %.
Finally, nano-aluminum has been known as a burn rate enhancer for propellants (typically double-base propellants) used in high pressure rockets. E.g., M. M. Mench, C. L. Yeh, and K. K. Kuo, Propellant burning rate enhancement and thermal behavior of ultra-fine aluminum powders (Alex), Proceedings of the 29th International Annual Conference of ICT, Karlsruhe, Federal Republic of Germany, pp. 30/1 to 30/15 (1998).
However, the burn rate and relatively low impetus of the foregoing references are not well-suited for meeting pretensioner performance requirements. Further, none of the foregoing references discloses a propellant employing a fuel comprising a solid solution of 5-AT, ADCA, and a superfine metal powder such as nano-aluminum.
In general, there continues to be a need in light of the foregoing teachings for a gas generant that has a beneficial combination of high impetus, high burn rate, high gas yield, low pressure exponent, high thermal stability, clean combustion products, and that is inexpensive to manufacture. Thus, there are continuing needs for gas generating compositions and safety devices produced therefrom that are less costly, more predictable in performance, and more compatible with applications such seatbelt pretensioners.